If you meet someone in a bar and hookup, chances are they have multiple characteristics that make you want to have sex with them. If you have an SO or FWB and have sex with them, there are probably multiple characteristics that make you want to have sex with them. (If in the above cases there is only one characteristic that makes you want to have sex with that person, then yes, that is fetishistic.)

In this case, they were putting out a (forgive me) cattle call for random women who had one specific criteria. They cared for nothing else, so long as the women were BBW. That is what makes it fetishistic, the fact that BBW is the only characteristic that matters.

There are three characteristics defined in BBW, not one. And the "beautiful" part of the three, while highly subjective, takes in a whole lot more than one characteristic all on its own.

And even aside from that, you appear to have come up with a definition of "fetish" that amounts to "anything or anybody a person desires sexually at the moment, even if they desire other things or people most of the time." I stand by my claim that this makes the definition apply to just about any sex act whatsoever.

ETA: I also notice that the issue of whether the headline would have been expected to make the story more "amusing" if it had stated that they'd specified people of "normal" weight, or people aged between twenty and thirty-five, hasn't been answered.